An ATP-activated endopeptidase from kidney and liver which cleaves parathyroid hormone (PTH) to fragments identical to those seen in vivo has been identified. This enzyme has an acid pH optimum and is found associated with a membrane fraction. It is readily released from membranes by freezing and thawing. It is not one of the known cathepsins and is not the ATP-dependent protease described by others in the cytoplasm of cells. It is proposed to purify and characterize this enzyme and study its role, specifically in parathyroid hormone metabolism, and generally in regulation of proteolysis by cells. An assay for the enzyme will be established which uses High Performance Liquid Chromatography (HPLC) to identify each of the three major products; C-terminal PTH peptides with N-termini at residues 35, 38 and 39. The enzymatic origin (more than one enzyme?) and preferred sequence of fragment production will be elucidated. Regulators other than ATP will be studied (CA++, guanosine tetraphosphate, C-AMP, others). The mechanism of ATP-activation will be investigated. The molecular weight, biochemical nature, and subunit composition of the enzyme will be determined. Antibodies against the enzyme will be generated and a radioimmunoassay established. Investigation of other possible substrates (other than PTH) will be conducted and investigation of whether the enzyme is specific for any particular type of protein will be done. The subcellular distribution of the enzyme and any precursor forms or degradation products of the enzyme will be studied by cell fractionation and immunoassay techniques. The tissue and cell distribution of the enzyme will be studied in eucaryotes and procaryotes. The effect of the calcium homoestatic regultors, vitamin D and parathyroid hormone on the activity and amounts of this enzyme will be studied. Finally, the fragments of PTH generated by this enzyme will be characterized and studied with regard to theri possible biological function.